a) Field of the Invention
The present invention relates to an objective lens system which is to be used with rays having wavelengths in the vicinity of 250 nm within the near ultraviolet region.
b) Description of the Prior Art
The objective lens systems designed for the rays having wavelengths in the vicinity of 250 nm in the near ultraviolet region are used as high resolution imaging lens systems, objective lens systems for studying living organisms with the rays within the ultraviolet region, and objective lens systems for researches and application of laser lights. Though some of these objective lens systems to be used for the above-mentioned purposes of application are designed as reflecting type, refracting type of objective systems are more advantageous for researches of living organism and laser lights in the ultraviolet region which require high resolution.
An objective lens of the refractive type designed for use with the rays in the ultraviolet region is described on page 689, JOSA, Vol 38. This objective lens system is of the glycerin oil immersion type, and has a high magnification, a large numerical aperture and favorably corrected spherical aberration.
Further, each of the objective lens systems for the ultraviolet rays disclosed by Japanese Patent Kokai Publication No. Hei 1-319719 and Kokai Publication No. Hei 1-319720 has a low magnification and a small numerical aperture, but comprises a small number of lens elements and has spherical aberration corrected relatively favorably.
The objective lens system described in the above-mentioned JOSA has remarkable curvature of field and a very narrow effective visual field, and is of the glycerin oil immersion type which cannot be used for researches of semiconductors or application of laser lights.
In the objective lens systems disclosed by Japanese Patents Kokai Publication No. Hei 1-319719 and Kokai Publication No. Hei 1-319720 which comprise the small numbers of lens elements, chromatic aberration and curvature of field are not corrected sufficiently in spite of the low magnifications and low numerical aperture thereof.